Power in AC Circuits

True or Real Power

In resistors, the power dissipation is caused by the opposition that the material offers to the flow of electrons. This electrical power is converted into heat and is referred as true power (P). The true power will not have any phase angle as there is no phase angle between voltage and current.

P = V x I

Reactive Power

In capacitors and inductors, the power is stored in the electro-magnetic field which has an effect on the electrons. This is known as reactive power (Q) and its formula similar to the true power formula but has a phase shift of 90^o as there is a phase angle between voltage and current.

Q = V x I

Apparent Power

Even though apparent power (S) is the sum of true power and reactive power, it is impossible to sum up as there is a phase angle between the two kinds of power. Thus the formula is derived using Pythagoras theorem.

Power Factor

The above three kinds of power form a triangle as shown in the following figure. Thus the laws for the calculation of triangles can be applied.

FIGURE